Both of mine are Therapy Dogs, and both were "home schooled".

The organization I work through is not delta or TDI, since we also have cats, rabbits, etc.
Your dog needs to be ok with loud noises that are typical in hospitals and nursing homes, clumsy/improper petting, and in case you do a team visit with other handler/dog teams, really needs to be ok with other dogs (it is embarrassing to meet in the lobby and have the dogs start to go after each other!).
But it is very rewarding. I find I have a hard time at one of the nursing homes I go to-it's me, I just have trouble seeing the physical and mental state of some of the patients.
There is also a demand for dogs who are good with kids. For example, there are programs where kids learn to read by reading to dogs!
Good luck to you!